At present, a handheld terminal device typically operates at multiple frequency bands, for example, frequency bands required for global system for mobile communication (GSM) and digital cellular system (DCS), an ultra-high frequency (UHF) required for a two-way radio, and a frequency required for global position system (GPS), so as to implement multiple functions or auxiliary functions. An antenna applied to the above handheld terminal device is a dual frequency antenna or a multiple frequency antenna, and most of the dual frequency antennas in the prior art adopt a double branch structure or a partial resonant structure. The dual frequency antenna with the double branch structure is composed of two antennas and the antennas are connected to one feeding point. Each of the two antennas has its resonance not affecting that of the other. Typically, a low frequency resonance is achieved by a helical structure, and a high frequency resonance is achieved by a whip structure. The length of the helical structure is one half of the wavelength (for the frequency of the low frequency resonance), and the length of the whip structure is one quarter of the wavelength (for the frequency of the high frequency resonance). The performance of the antenna operating at the two frequencies is similar to that of a half-wave dipole.
A dual frequency antenna with the partial resonant structure may achieve a dual frequency resonance by changing a pitch of a part of the helical structure, and the length of the part in which the pitch is changed is a resonant length at the other required frequency. The performance of the antenna operating at two frequencies is similar to that of the half-wave dipole. Most of the existing external dual frequency antennas are achieved by the partial resonant structure. In the helical structure, the high frequency resonant part is placed on the bottom of the coil to form a lower frequency resonance together with another part. The particular structure is shown in FIG. 1.
The above-mentioned two kinds of external helical dual frequency antennas are operated at UHF/VHF (Ultra High Frequency) & GPS frequency bands, and the resonance is formed by changing a pitch of a part of the coil or placing a whip antenna at the bottom of the helical, in which the length of the whip antenna is one quarter of the wavelength. This design is relatively simple, and for the GPS frequency band, the performance of the antenna is more centralized on the lower hemisphere. There is a large recess in the upper hemisphere (the part directed to the sky) required by the GPS, and therefore this design has a poor performance and is adverse to the reception of a GPS signal.
Furthermore, if the dual antenna is designed for the VHF frequency band, there is huge difference (approximately 10 frequency multiplication) between the two frequencies, and small deviation of the VHF frequency may cause huge difference of the GPS signal.